Holograms have come into wide usage as decorative indicia due to their unique visual appearance. In addition, the difficulty in making and reproducing holograms has made them a common authentication feature on security items such as credit cards, driver's licenses and access (identification) cards. Holograms have also been used as security features on products in order to prevent piracy or counterfeiting.
The most common method of creating a hologram is to create a grating pattern in a surface so that particular structures become visible upon diffraction of light in the grating. U.S. Pat. No. 3,578,845 to Brooks et al. describes how diffraction gratings are typically generated. Typically, the diffraction patterns are embossed into a thermo-formable substrate such as an embossable polymer film. This process is performed by pressing a heated stamp made from a hard material to engrave the desired grating from the contact surface of the stamp onto the embossable substrate.
Diffraction requires that the medium the grating is made of and the media bordering the grating have a difference in optical index. The larger this difference is, the brighter the diffraction will appear. To create highest diffraction, full reflective materials such as aluminum, copper or gold are thin film coated onto the surface of the grating.
Alternately, the grating is coated with a thin film of transparent material having a high refractive index (HRI) such as Zinc Sulfide (ZnS), Iron Oxide (Fe2O3), Lead Oxide (PbO), Zinc Selenide (ZnSe), Cadmium Sulfide (CdS), Titanium Oxide (TiO2), Lead Chloride (PbCl2), Cerium Oxide (CeO2), Tantalum Oxide (Ta2O5), Zinc Oxide (ZnO), Cadmium Oxide (CdO), Neodymium Oxide (Nd2O3) or Aluminum Oxide (Al2O3). Substrates coated with a transparent HRI coating are often used for security applications such as identification or access cards, where it is desired that information positioned behind the hologram remains visible to the unaided eye.
While the grating can be embossed into the substrate material with a stamp, a more common, economical method, is the use of continuous embossing systems. Such embossing system are described for example in U.S. Pat. Nos. 4,913,858 and 5,164,227, both to Miekka et al. In these methods the grating structure is engraved into the surface of a roll, which continuously presses its surface pattern into the web type substrate passing between the embossing roll and a backside roll. In order to obtain the grating in the substrate's surface, the thermo-formable layer on this surface is heated. This can be achieved either by preheating the substrate to the required temperature, or by heating the embossing roll.
Commonly the art differentiates between “Soft Embossing” and “Hard Embossing”. Soft Embossing describes the process where the embossing is performed before a high reflective index (HRI) layer is applied. “Hard Embossing” is performed by creating the grating through the HRI layer. While Hard Embossing is done on substrates coated with “soft” metals like aluminum, copper or gold, it is not typically done through semi-transparent reflection enhancement HRI layers like ZnS or TiO2. Semi-transparent layers are coated thicker than metal coatings in order to achieve the brilliance and reflectivity typically desired. The typical coating thickness for ZnS for example is about 400–600 Angstroms (40–60 nm), while aluminum is typically coated with about 200 Angstroms or less. In addition, semi-transparent coatings are much harder than metal coatings. ZnS has a Mohs hardness of 4.5 Mohs, compared to a Mohs hardness of 2.75 for aluminum. These two factors would require higher embossing pressure on standard substrates and increase the wear on the embossing shims.
A common process for producing an embossable web type substrate such as an embossable polyethyleneterephthalate (PET) or polypropylene (PP) is to apply a thermo-formable coating onto at least one surface of the polymer film. This process is done off-line, i.e. after manufacturing of the polymer web. Embossable coatings typically are applied either as a water-based or as a solvent-based solution using coating systems well known in the art such as roll coating, gravure coating, air knife coating or rod coating, among others.
The coatings are dried in hot oven systems, designed to drive out the moisture and solvents and to lock the coating into a coherent structure. It is common, though, that an excessive amount of moisture or solvent is retained in the coating. HRI coatings, however, are typically applied using vacuum deposition processes such as physical vapor deposition. The exposure of the embossable coatings to a vacuum causes the remaining moisture or solvent to evaporate, a process called “outgassing”. Outgassing is an unwanted reaction as it hinders the deposition of the HRI coating, causing uneven deposition of the HRI material and rendering the material useless for commercial application. Embossing done prior to vacuum coating exposes the coating to additional heat and pressure of the embossing process, improving the removal of entrapped moisture and solvents. This is an additional reason why the transparent HRI coating is typically not done until after application of the embossing.
As is described in U.S. patent application Ser. No. 10/087,689, filed Mar. 1, 2002, and Ser. No. 10/206,453, filed Jul. 26, 2002, the both of which hereby are incorporated in their entirety by reference, it would be desirable to provide an embossable film structure that is made at the point of film manufacturing. As is pointed out, however, embossable surfaces produced through a co-extrusion process need to have many of the same characteristics of the base film. Therefore, inherent viscosity (IV), melt strength, melt viscosity and the like are important parameters for getting the co-extruded layer through the film making process.
Typical materials that can survive this process are often analogs of the base film material itself. These materials suffer the problem of having low crystallinity and are, therefore, heat sealable. A heat sealable material will often stick to the embossing shim, rendering the embossed texture of little commercial quality. Such a co-extruded layer, however, would also be free of moisture or solvent, thus eliminating the above described problems of outgassing.
In addition, it would be desirable to have an embossable multi-layer film that is coated with a transparent HRI coating that can be embossed through the HRI coating, directly accepting holographic texture and presenting a good image after lamination. Such a material would offer higher flexibility for the production of semi-transparent holograms. The nature of the HRI coating process requires that specific minimum lengths have to be coated. This length requirement is not given if HRI coating is applied prior to embossing. Such a material would allow for shorter, volume limited production of specific holograms. Especially in the area of high security holograms it would reduce the high risk requirement of moving high security holograms between facilities.